J. Phys.Chem. Thermochemistry of 2- and 3-Acetylthiophenes: Calorimetric and Computational Study María Victoria Roux*, Manuel Temprado, Pilar Jiménez, Rafael Notario Instituto de Química Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain James S. Chickos Department of Chemistry and Biochemistry, University of Missouri- St. Louis, OneUniversity Boulevard, St. Louis, Missouri 63121-4499, USA Ana Filipa L. O. M. Santos, Manuel A. V. Ribeiro da Silva Centro de Investigação em Química, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal ∗ to whom correspondence should be addressed: Maria Victoria Roux e.mail: [email protected] 2 Abstract The relative stabilities of 2- and 3- acetylthiophene have been evaluated by experimental thermochemistry and the results compared to high-level ab initio calculations. The enthalpies of combustion, vaporization and sublimation were measured by rotating-bomb combustion calorimetry, Calvet microcalorimetry, correlation gas chromatography and Knudsen effusion techniques and the gas-phases enthalpies of formation, at T = 298.15 K, were determined. Standard ab initio molecular orbital calculations at the G2 and G3 levels were performed, and a theoretical study on the molecular and electronic structures of the compounds studied has been conducted. Calculated enthalpies of formation using atomization and isodesmic reactions are compared with the experimental data. Experimental and theoretical results show that 2- acetylthiophene is thermodynamically more stable than the 3-isomer. A comparison of the substituent effect of the acetyl group in benzene and thiophene rings has been carried out. 3 1. Introduction Thiophene is a chemically stable compound, readily available, and the chemistry of thiophene and its derivatives has been a constant matter of investigation. Several books and reviews are available on the chemistry of thiophenes.1 It is the simplest representative of an aromatic structure bearing sulfur. Thiophene obeys the 4n + 2 π electron rule, and it is generally considered to be aromatic.2 Its structure can be assumed to be derived from benzene by the replacement of two annular CH groups by sulfur. The sulfur atom in this five membered ring acts as an electron donating heteroatom by contributing two electrons to the aromatic sextet, and thiophene is thus considered to be an electron-rich heterocycle. Thiophene derivatives comprise a significant portion of the organosulfur compounds in petroleum and in other products from fossil fuels, being obtainable as by-products of petroleum distillation.3 Different studies related to their biodegradation4 and catalytic dehydrodesulfuration5 have been carried out. Thiophene-based compounds have also found widespread use in modern drug design,6 biodiagnostics,7 electronic and optoelectronic devices,8 and conductive and electroluminescent polymers.9 Also several reviews of various aspects of thiophene coordination and reactivity in transition metal complexes have been reported.10 Over the past years, we have been involved in a thermochemical study of the energetics of thiophene derivatives.11-16 A development of an understanding of the structural effects on thermodynamic stabilities reflected in the gas phase enthalpies of formation of the compounds studied has been of fundamental interest to us. Substituents on the five membered ring of thiophene have a different relationship to one another than similar substituents in benzene. Our work has focused mainly on the energetic differences between isomers containing different functional groups in positions 2 and 3, and the substituent effects in thiophene relative to corresponding benzene analogues. In a previous article we reported the 4 thermochemistry of 2- and 3- thiophenecarboxylic acids.11 In general, the -COOH group of thiophenecarboxylic acids acts much the same way as in other organic compounds. We showed previously, that 3-thiophenecarboxylic acid is slightly more thermodynamically stable than 2-thiophenecarboxylic acid with an isomerization enthalpy of 2.6 kJ mol–1. In comparison with benzene and its analogous substituted derivative, benzoic acid, we found that the 2-isomer is destabilized by 2.4 kJ mol–1 as a result of the through-space interaction between the oxygen of the hydroxyl group and the sulfur atom.13 On the basis of a similar comparison with the result obtained for 3-thiophenecarboxylic acid, –0.2 kJ mol–1, we concluded that there are not significant differences between the substituent effects produced by a carboxyl group in benzene and the 3-position in thiophene.13 Similarly, comparing the relative stability obtained by substituting two H atoms by COOH groups in the 2,5- position of thiophene and at the 1,4- positions of benzene results in similar energetic effects.15 Moreover, the experimental difference between the enthalpies of formation of 2- and 3- thiopheneacetic acid, methyl esters, 0.9 ± 2.1 kJ mol-1, is too close to call.16 These results are consistent with the expectation that in the absence of substantial steric and electronic effects (the through space interaction observed in 2-thiophenecarboxylic acid), alkyl substituents, less capable of resonance interactions, are likely to have a smaller effect on relative stabilities than observed for carboxyl groups. The purpose of the present work is to study the energetics of the 2- and 3- acetylthiophenes whose formula is presented in Figure 1. The approach selected is a combination of experimental determination of the enthalpies of formation and high-level ab initio calculations. Figure1 near here 2. Experimental Section 5 2.1 Material and Purity Control. Commercial samples of 2- and 3- acetylthiophene were obtained from Lancaster at a stated mass fraction of 0.99 and 0.98 respectively. To obtain a dry and pure sample of 2-acetylthiophene, the commercial compound was dried over 4A molecular sieves before and after fractional distillation in a packed column under reduced pressure. This procedure was repeated three times. 3-Acetylthiophene was purified by crystallization twice from petroleum ether and then it was carefully dried under vacuum. To assess the purity of the compounds, a standard gas chromatograph with a 60m BPX70 fused silica SGE capillary column, was used. The purity of 3-acetylthiophene was also assessed by DSC by the fractional fusion technique.17 The water content of liquid 2-acetylthiophene was determined using a Karl Fisher instrument (Metron 682) and by GC-MS. The purified 2-acetylthiophene had 0.396% of the 3- isomer present as an impurity, and a water content of 0.009 %. No other impurities were detected. The mol fraction of 3-acetylthiophene was better than 0.999. 2.2 Experimental Determination of the Enthalpies of Formation in the Gas Phase. o The enthalpies of formation in the gas phase of 2- and 3-acetylthiophene, ∆ f H m (g), at T = 298.15 K, were determined from the experimental values of their standard enthalpies of o formation in the condensed state, ∆ f H m (cd), and the standard phase change enthalpies, g o g o ∆ l H m and ∆cr H m , the 2- isomer being a liquid and the 3-isomer being a solid at this temperature. 2.2.1. Combustion Calorimetry. The enthalpies of formation in the condensed state were determined from combustion calorimetry using two isoperibol combustion calorimeters equipped with rotating bombs. The enthalpy of combustion of 2-acetylthiophene was measured at the University of Porto and the enthalpy of combustion of 3-acetylthiophene at the Instituto de Química Física Rocasolano in Madrid. Details of the techniques and procedures used have been previously described.18,19 6 The samples of 2-acetylthiophene were contained in sealed polyester bags made of o Melinex (0.025 mm of thickness) with massic energy of combustion ∆cu = −(22902 ± 5) J⋅g−1,20 a value which was confirmed in Porto laboratory. The mass of Melinex used in each experiment was corrected for the mass fraction of water (w = 0.0032), and the mass of carbon dioxide produced from its combustion was calculated using the factor previously reported.20 The energy of combustion of the 3- isomer was determined by burning the solid samples in pellet form enclosed in polyethylene bags. The empirical formula and massic energies of -1 combustion of the polyethylene were C0.968H2.000 and -(46339.3 ± 6.6) J g respectively, and were determined in Madrid. The energy equivalent of the Spanish calorimeter, ε(calor), was determined under the same conditions in which the experiments were performed using the combustion of benzoic acid, NIST standard reference sample 39j. The value of the energy equivalent of the calorimeter (ε(calor, Madrid) = {28871.08 ± 0.90 (0.003 %)} J·K-1) was determined from ten calibration experiments and this value was used in computing the results of the combustion experiments performed in Madrid. The energy equivalent of the calorimeter of the Portuguese system, ε(calor, Porto) = {20369.0 ± 2.3 (0.011%)} J⋅K−1, was determined without rotation of the bomb as previously described,21 from an average of ten calibration experiments.18 The calorimeter was calibrated using benzoic acid, NBS thermochemical standard 39i, having a massic energy of combustion under bomb conditions of –(26434 ± 3) J·g-1 (using an average mass of 3965.0 g of water added to the calorimeter); the quoted uncertainty is the standard deviation of the mean. 3 Under the conditions of the experiments (Tth = 298.15 K; Vbomb (Madrid) = 0.260 dm ; 3 i 3 Vbomb (Porto) = 0.329 dm ; p gas = 3.04 MPa; and 10.00 (Madrid) or 15.00 cm (Porto) of water added to the bomb), no carbon (soot), CO, or SO2 were detected. Corrections for nitric acid formation, from traces of N2 remaining inside the bomb, were −1 −3 based on −59.7 kJ⋅mol for the molar energy of formation of 0.1 mol⋅dm HNO3 (aq) from 7 22 O2, N2, and H2O (l).